The present invention relates to an optical recording medium and in particular to an optical recording medium excellent in the repetitive overwrite characteristic.
Optical recording media such as CDs and DVDs have been in widespread use as recording media for recording digital data.
As the data recording system for optical recording media, in general, data to be recorded is modulated to a predetermined length as a recording mark formed along a track provided on an optical recording medium. For example, on a DVD-RW as a type of an optical recording medium that allows data overwriting by the user, recording marks corresponding to 3T through 11T or 14T (T representing one clock cycle) are used and such a mark is formed onto a recording film included in an information layer along a track provided on the optical recording medium, thereby recording data.
In this way, in case data is recorded by forming recording marks onto a recording film included in an information layer of a data rewritable optical recording medium, a laser beam is irradiated onto a recording film along a track provided on the optical recording medium. In this process, a phase change material in crystalline state included in the recording film turns into an amorphous state thus forming an amorphous area having a predetermined length onto the recording area. The amorphous area thus formed is used as a recording mark.
That is, in case data is recorded onto a recording film included in an information layer of a data rewritable optical recording medium, a laser beam having a power set to a writing power Pw at a sufficiently high recording level is irradiated onto the recording film. An area of the recording film where the laser beam is irradiated is headed above the melting point of a phase change material and thus melts. Next, a laser beam having a power set to a base power Pb at a sufficiently low level is irradiated onto the recording film, which rapidly cools the melted area. As a result, the phase change material included in the area of the recording film changes from the crystalline state to the amorphous state, and a recording mark is formed onto the recording film thus recording data.
In case a recording mark formed on the recording film included in the information layer of a data rewritable optical recording medium is to be erased in order to erase data recorded on the recording film, a laser beam having a power set to an erasing power Pw exceeding the base power Pb is irradiated onto the recording film. This heats an area where the recording marks is formed above the crystallization temperature of the phase change material, followed by gradual cooling of the heated area of the recording film. As a result, the phase change material included in the area of the recording film where the recording marks is formed changes from the amorphous state to the crystalline state, the recording marks is erased and data is erased.
Thus, by modulating the power of a laser beam irradiated onto a recording film included in the information layer of a rewritable optical recording medium to one of a plurality of levels corresponding to a writing power Pw, base power Pb and an erasing power Pe, it is possible to form a recording mark onto a recording film for data recording as well as erase the recording marks formed onto the recording film while forming a new recording mark thereon to perform direct overwriting of data using new data.
In this way, in case the data recorded onto a recording film included in the information layer of a rewritable optical recording medium is to be overwritten directly, nit is preferable to rapidly crystallize the phase change material forming a recording mark in the amorphous state to erase the recording mark. To this end, in general, a recording film included in the information layer of a rewritable optical recording medium is formed by a phase change material having a high crystallization velocity. As optical recording media including a recording film formed by a phase change material having a relatively high crystallization velocity, there are known an optical recording medium including a phase change recording film where one or more of the elements including Ag, Au, Cu, Zn, B, Al, Ga, In, Si, Ge, Sn, Pb, N, P, Bi, La, Ce, Gd and This added to an essential element SbTe (refer to JP-A-2000-322740) and an optical recording medium including a recording film represented by a composition formula GeαSbβ, where α and β are atomic ratios, 5≦α≦40 and 60≦α≦95 (refer to JP-A-2004-195742).
In case data is directly overwritten into a data-recorded area for the first time after the data is recorded onto an optical recording medium described in JP-A-2000-322740 followed by prolonged storage under a high temperature, the resulting jitter is considerably worsened with the storage characteristic degraded excessively.
While the optical recording medium described in JP-A-2004-195742 shows a better storage characteristic than that described in JP-A-2000-322740, it is recently requested that various characteristics of an optical recording medium be improved. In order to fully respond to such requests, it is desired to further upgrade the storage characteristic of an optical recording medium.
In case data is overwritten 1000 times repeatedly onto an optical recording medium described in JP-A-2000-322740 or JP-A-2004-195742, the jitter is worsened thus degrading the repetitive overwriting characteristic.